3D visualisation of the experiment: The iron atoms (red) are arranged in the X-ray waveguide in a layer about two atomic layers thick. The X-ray pulse (white) comes from the rear left, whereupon the atomic nuclei together emit a single photon (green waveform). (Credit: Leon M. Lohse)
“The whole is more than the sum of its parts” – even in quantum physics. Quantum objects, even simple ones such as atoms with only two possible energy states, can behave completely differently in a group than they do on their own. The behaviour of a whole depends on how the individual parts are arranged and connected. At DESY´s X-ray lightsource PETRA III, team of researchers from the University of Göttingen, DESY and other institutions have coupled a large ensemble of atomic nuclei of the iron isotope 57Fe via an X-ray waveguide for the first time. They were able to observe how the atomic nuclei collectively emit a single X-ray photon into the waveguide and how it propagates there. The measurements agree with a theoretical model that was developed with researchers from the Universities of Erlangen-Nürnberg and Würzburg. The study opens up a new field of research that combines classical nuclear resonance experiments with modern quantum optics and waveguide technologies. It was published in the journal Physical Review Letters.
Waveguides guide oscillations such as light, sound or radio waves in a controlled direction. They are used in technical applications such as ultrasound devices, radar systems and quantum computers. Glass fibres are familiar from everyday life. In the laboratory, waveguides are used, for example, to bring individual atoms into interaction with the help of light. Until now, this was only possible with long-wave radiation. In experiments at PETRA III, many atomic nuclei of the isotope 57Fe have now been successfully transferred from their energetic ground state to an excited state using hard X-rays, very short-wave radiation. 57Fe, also known as iron-57, is contained in about two per cent of natural iron.
The researchers first excited the 57Fe atomic nuclei with short flashes of X-ray photons. Such a transition between energy states, known as nuclear resonance, is only triggered if the energy of the radiation matches the energy difference in an atomic nucleus exactly. Each X-ray pulse, consisting of around 1000 photons, contained on average less than one energy-matching photon. When atomic nuclei are excited together, they influence each other and do not fall back to their ground state independently, but collectively. The researchers measured this joint decay, in which the atomic nuclei release energy in the form of photons. Depending on the arrangement of the atomic nuclei, characteristic temporal signals emerged that could be analysed in detail for the first time.
“A major technical challenge of the experiment was the preparation of X-ray waveguides, which enabled both good coupling of the atomic nuclei over large distances and allowed the strong X-ray pulses to be bundled with low loss – without overloading the sensitive measurement electronics,” explains first author Leon M. Lohse, who completed his PhD at the Institute of X-ray Physics at the University of Göttingen and is now a researcher at the University of Hamburg. To this end, the team in Göttingen and at DESY fabricated millimetre-long waveguides from nanometre-thin layers in several steps. “Many new ideas have emerged from these initial experiments,” says Lohse. For example, the X-ray source PETRA IV planned at DESY could be used to feed a thousand times more photons into the waveguides. This would make much more complex experiments possible, which would require the interconnection of different nuclear ensembles via several waveguides.
(Partly from DESY News)
Reference:
Lohse, Leon M. et al. Collective Nuclear Excitation and Pulse Propagation in Single-Mode X-Ray Waveguides, Physical Review Letters (2025). DOI: 10.1103/r2hf-9qn9
